bpo-38322: Fix gotlandmark() of PC/getpathp.c (GH-16489)

Write the filename into a temporary buffer instead of reusing prefix.
The problem is that join() modifies prefix inplace. If prefix is not
normalized, join() can make prefix shorter and so gotlandmark()
does modify prefix instead of returning it unmodified.

PC/getpathp.c

@@ -315,15 +315,13 @@ canonicalize(wchar_t *buffer, const wchar_t *path)
    'prefix' is null terminated in bounds.  join() ensures
    'landmark' can not overflow prefix if too long. */
 static int
-gotlandmark(wchar_t *prefix, const wchar_t *landmark)
+gotlandmark(const wchar_t *prefix, const wchar_t *landmark)
 {
-    int ok;
-    Py_ssize_t n = wcsnlen_s(prefix, MAXPATHLEN);
-
-    join(prefix, landmark);
-    ok = ismodule(prefix, FALSE);
-    prefix[n] = '\0';
-    return ok;
+    wchar_t filename[MAXPATHLEN+1];
+    memset(filename, 0, sizeof(filename));
+    wcscpy_s(filename, Py_ARRAY_LENGTH(filename), prefix);
+    join(filename, landmark);
+    return ismodule(filename, FALSE);
 }